Saints Athanasius and Cyril were Archbishops of Alexandria. These wise teachers of truth and defenders of Christ's Church share a joint Feast in recognition of their dogmatic writings which affirm the truth of the Orthodox Faith, correctly interpret the Holy Scripture, and censure the delusions of the heretics.
St Athanasius took part in the First Ecumenical Council when he was still a deacon. He surpassed everyone there in his zeal to uphold the teaching that Christ is consubstantial (homoousios) with the Father, and not merely a creature, as the Arians proclaimed. This radiant beacon of Orthodoxy spent most of his life in exile from his See, because of the plotting of his enemies. He returned to his flock as he was approaching the end of his life. Like an evening star, he illumined the Orthodox faithful with his words for a little while, then reposed in 373. He is also commemorated on May 2 (the transfer of his holy relics).
St Cyril was the nephew of Patriarch Theophilus of Alexandria, who educated him from his youth. He succeeded to his uncle's position in 412, but was deposed through the intrigues of the Nestorian heretics. He later resumed his See, however. St Cyril presided at the Third Ecumenical Council in 441, which censured the Nestorian blasphemy against the Most Holy Theotokos. His wise words demonstrated the error of their false doctrine. St Cyril departed to the Lord in the year 444, and is also commemorated on June 9 (the day of his repose)."
Apolytikion of Sts. Athanasios and Cyril, Patriarchs of Alexandria in the Third ToneShining forth with works of Orthodoxy, ye quenched every false belief and teaching and became trophy-bearers and conquerors. And since ye made all things rich and with true piety, greatly adorning the Church with magnificence, Athanasius and wise Cyril, ye both have worthily found Christ God, Who doth grant great mercy unto all.
Kontakion in the Fourth ToneO great Hierarchs of piety and brave champions of the Church of Christ, you watch over all who sing, "Save us who in faith honor you, O Compassionate."
Source: full-of-grace-and-truth.blogspot.com